Definitions have been provided to help with a general understanding of network transmissions and are not meant to limit their interpretation or use thereof. Thus, one skilled in the art may substitute other known definitions or equivalents without departing from the scope of the present invention.
Simplex Communication: Refers to transmission in only one direction. Thus, simplex refers to one-way communications where one party is the transmitter and the other party is the receiver. An example of simplex communications is a simple radio, wherein users are able to receive data from stations but are unable to transmit data.
Datagram: A portion of a message transmitted over a packet-switching network. One key feature of a packet is that it contains the destination address in addition to the data. In IP networks, packets are often called datagrams.
Push: Push refers to sending data to a client. The World Wide Web (WWW) is based on a Pull technology where the client browser must request a Web page before it is sent (pushed). Broadcast media, on the other hand, utilize Push technologies because information is sent (pushed) out regardless of whether anyone is tuned in.
Increasingly, companies are using the Internet to deliver information Push-style. The most widely used Push technology is e-mail. This is a Push technology because one receives mail whether they ask for it or not; that is, the sender pushes the message to the receiver.
Pull: Pull refers to requesting data from another program or computer. The opposite of Pull is Push, where data is sent without a request being made. The terms Push and Pull are used frequently to describe data sent over the Internet. As mentioned earlier, the WWW is based on Pull technologies, where a page isn't delivered until a client requests it. Increasingly, however, information services are harnessing the Internet to broadcast information using Push technologies. A prime example is the PointCast Network™.
Presently, receivers for radio transmissions generally operate using analog VHF transmission. Digital audio broadcast (DAB) provides a method and system for the simplex (one-way) transmission and reception of high-quality audio (music and/or speech) and ancillary digital data content by radio frequency (RF) signals. The RF signals are generated in and broadcast from a transmitter system, and received by any of multiple receiver systems, which may be a consumer electronics device (such as a car radio receiver). Typically, the transmitters are satellites or land-based, or a combination of space and terrestrial equipment. In general, terrestrial transmission is required for adequate mobile receiver performance in areas with dense natural and/or man-made structures, because satellite reception typically requires line-of-sight (LOS) propagation due to large propagation path signal losses.
As with any digital RF signal, the RF signal for DAB represents digital (bit) information that is encoded in the signal generated in the transmitter system by a modulation method. Unlike conventional analog FM modulation, the information represented by the digital signal (e.g., digitized speech, music, and/or data) is typically unrelated to the characteristics of the transmitted signal. A primary goal of DAB is to eventually supplant the existing commercial analog radio broadcast network (i.e., AM-band and FM-band). Since the primary function of the existing AM-band and FM-band is to provide audio services in the form of music and/or speech, it is presumed that a significant fraction of the encoded digital data represents one or a plurality of digital audio signals.
In many circumstances, the quality of the digital audio signal recovered in the DAB RF signal receiver can be expected to be of higher quality than the audio signal recovered from the conventional analog frequency-modulated (FM) signal. Typically, the digital audio signal has a higher signal-to-noise ratio (SNR), a larger audio bandwidth, and improved stereo separation (i.e., spatial fidelity) when compared to conventional FM-band reception. For example, conventional analog FM-broadcast signals have a recovered audio bandwidth of about 15 kHz. While 15 kHz audio bandwidth is substantially greater than the audio bandwidth received for commercial AM-band radio broadcast (i.e., 535 kHz-1705 kHz, which is typically less than 10 kHz), 15 kHz is still less than the bandwidth of pre-recorded music on the compact-disc (CD) format, which is about 20 kHz).
The improved audio bandwidth advantage of DAB when compared to conventional analog FM-band reception is generally desirable. However, the most significant advantage of a DAB signal when compared to an analog FM-band signal is its greater immunity to various forms of distortion and interference at the DAB receiver. Distortion and interference in the recovery of an analog FM-band signal causes various undesirable artifacts in the audio signal at the receiver. Examples include static noise, hiss and hum, and clicks. In certain circumstances, background noise limits the quality of the recovered signal. Background noise may be generated by galactic, atmospheric, thermal, and man-made sources, as in the example of an automobile engine-generating interference in a car radio.
Background noise is particularly noticeable in a received analog FM-band signal when there are pauses or quiet passages in the transmitted audio program. The process of analog FM modulation typically improves the robustness of the recovered signal when compared to other analog modulation methods such as amplitude modulation as used in the AM broadcast band. This improvement results from frequency modulation increasing the occupied bandwidth of the signal, thereby causing “processing gain”. However, the benefits of the analog FM processing gain are substantially eliminated when the source audio signal is silent or quiet.
Background noise is typically distinguished from distortion, which has two primary sources. The first source of signal distortion is related to the propagation characteristics of the RF signal. A common example is multipath distortion, where the effects of reflection, refraction, and attenuation can cause a single transmitted signal to travel to a receiver along several different paths. The signals from separate paths naturally have different characteristics which may interfere with each other. The deleterious effects of multipath are a result of the relatively high RF carrier frequencies, which are characteristic of FM-band frequency range, and the use of omnidirectional receiving antennas, especially for radio receivers in vehicles. Multipath propagation results from the presence of specular and/or diffuse reflectors in or about the propagation path between the transmitter and the receiver. As a result, multiple signals with varying delay, phase, amplitude, and frequency are received, these signals corresponding to different propagation paths. In general, the deleterious effects of multipath are attenuation of the RF signal due to destructive coherent interference between paths, dispersion in the received RF signal due to frequency selective characteristic of multipath, and intersymbol interference between adjacent signal baud intervals. Multipath is typically mathematically modeled as a deterministic linear sum or stochastic function of the transmitted signal and reflections, with background noise modeled independently in the summation.
The second common source of signal distortion is interference caused by the presence of other RF signal sources with similar frequencies, including FM-band transmitters. In circumstances when there are many transmitters, for example, in large urban areas, inter-station interference due to other transmitters operating at the same or similar RF frequencies may be the primary cause of signal degradation, except for weak-signal conditions at substantial distances from the transmitter or unusual reception circumstances (e.g., signal shielding in a tunnel).
Because broadcast transmitters may emit high-power RF signals, their operation is often dictated by government agency-enacted rules and regulations that are intended to prevent interference between stations. However, there are sometimes specific exceptions granted that permit transmitters to avoid strict adherence to these rules. Furthermore, the presence of noise and multipath distortion may cause reception conditions that are substantially degraded when compared to the nominal conditions that are presumed to exist according to the rules. In the United States, the Federal Communications Commission (FCC) determines the rules and regulations governing FM-band broadcast.
The general goals of a DAB system are independent of the specific implementation of the DAB system. The commonly intended purpose of DAB is to provide a simplex communication system, which conveys digital audio and ancillary data in the form of RF signals from one or a plurality of transmitters to one or a plurality of receivers. It is desirable that the determination (i.e., demodulation and decoding) of the transmitted DAB signal at the receiver is less degraded by the effects of noise, multipath, and interference compared to the reception of conventional FM-band broadcast signals for DAB and FM signals with equivalent coverage. The quality of the recovered digital audio signal represented by the DAB RF signal when unimpaired is typically equal or nearly as high as pre-recorded CDs. However, the direct digital transmission of data in the CD format is inefficient because of the high data rate that is required for faithful CD representation, which is greater than one million bits per second for stereo signals. In a DAB system, the audio—speech and/or music—is source compressed, thereby reducing the required data throughput substantially. Source compression accomplishes a reduction in the required bit rate for audio information which is at or below a threshold of perception and whose absence will not typically be noticed by the listener.
In a conventional DAB system, the data representing the compressed audio signal is combined with ancillary data and then error-correction encoded, modulated, and emitted as an RF signal at the DAB transmitter. In the corresponding conventional DAB receiver system, the received RF signal is demodulated, error correction decoded, and the effect of the source-compression is reversed to generate a conventional digital audio signal, for example, in the form of a pulse-code modulated signal. The design and implementation of an audio compression and decompression (codec) algorithm is a complex process, but several methods which are suitable for DAB are well known in the art.
An important characteristic of DAB systems is the ancillary data capability. The ancillary data most often transmitted and/or that is desired to be transmitted in a DAB signal is typically unrelated to the digital audio signal. The ancillary data permits broadcasters to increase the range of services, which are provided to listener receivers, including services other than audio services. This characteristic of DAB systems is particularly attractive since the improvement in received audio quality over conventional FM-band reception brought about by the implementation of a DAB system alone may be insufficient to justify the costs associated with widespread conversion from conventional analog FM-band modulation to a DAB system.
Currently, analog FM-band stations may transmit limited associated data services along with analog FM signals, through analog FM subcarrier signals. The subcarrier signals are known as “SCA” signals because of their authorization by the Subsidiary Communication Authorization granted by the FCC. SCA signals are combined with the conventional FM audio signal matrix at baseband so that, unlike DAB, the SCA signals are combined with the conventional FM audio signal matrix at baseband so that, unlike DAB, the SCA signal is a component of the transmitted FM signal. A disadvantage of SCA signals is that their bit rate throughput is relatively low, typically less than 30 kbit/s. Furthermore, the SCA signal is susceptible to the effects of interference and distortion in the same manner that analog FM-band audio reception is degraded because demodulation of the SCA signal typically requires conventional analog FM modulation as the first step in recovering the SCA signal. For a specific frequency deviation, the robustness brought about by the use of FM modulation diminishes as the baseband modulating frequency is increased, as the FM processing gain for the embedded SCA signal is relatively small.
For DAB, a preferred characteristic of the ancillary data associated with the digital audio signal is that the recovered ancillary data signal is of a reasonable bit rate throughput. For example, the bit rate throughput is at least 64 kbit/sec, and at least as robust to the effects of interference and distortion as the remainder of the digital data, including the digital audio signal, in the DAB signal. While audio compression algorithms may operate correctly with decoded bit error rates as high as 1×10−5 because transient errors may not be detected by the human hearing process, ancillary data may require a much lower decoded bit error rate, typically less than 1×10−7. The range of services known in the prior art which may be implemented with the DAB ancillary data include traffic, weather, road conditions, and emergency information; subscriber services such as paging and specialized newscasts such as stock quotes and sports scores; low-bit rate video and still pictures; electronic-mail and Internet broadcast; satellite navigation data; and duplex communication and electronic commerce with return data-channel accomplished, for example, by cellular telephony.
A fundamental way to characterize a DAB system is by the method which is used to generate the RF signal at the transmitter and to determine the RF signal at the receiver. There are different RF modulation and demodulation methods which may be used to implement a DAB system. One characteristic in distinguishing between methods in whether or not the proposed DAB system implementation requires a new frequency allocation other than the existing AM and FM broadcast bands. The high-power RF signal emissions for DAB will typically be regulated by government agencies in order to mitigate RF interference. Control of the frequency allocation through specific channel frequency assignments and RF emission limits (i.e., average power or power spectral density) prevents interference from other RF signals sources into the desired DAB signal and vice versa. Since the DAB receivers are expected to be a large-scale consumer electronics application, standardized frequency allocations must also be considered in order to develop economically viable receiver systems.
However, in order to improve the transmission quality, digital processes are frequently used, such as digital satellite radio (DSR). Thereby, a transmission signal is transmitted from the transmitter via a satellite to the receiver. In order not to equip every single receiver with relatively large and expensive antenna and a first low-noise mixer for the satellite radio, a DSR signal may be fed from a central terrestrial receiver, and also over broadband cable networks to the receiver.
A digital receiver is used to achieve the best possible reception quality of a digital transmission signal. Such a digital receiver is known from German Patent DE 34 28 318 A1. In this receiver, the analog reception signal is mixed down into an intermediate frequency signal by a mixer, is band-limited with an analog bandpass filter, sampled, analog-to-digital converted, and multiplied with complex signal of an oscillator in at least one digital signal processor. In this manner, the quadrature components of the reception signals are generated in the baseband.
The disadvantage in this receiver is the expensive processing of the analog-to-digital converted signal. At least one digital signal processor is required for performing a multiplication with a complex oscillator signal.
A digital demodulator for frequency-modulated signals is disclosed in European publication EP 0201758 B1, which enables compensation of multipath reception. In this demodulator, the reception signal is first mixed down into an intermediate frequency signal, band-limited and analog-to-digital converted. Thereafter, the digital signal is fed to a cascading equalizer, which performs a reduction of the reflections contained in the input signal.
This demodulator is disadvantageous in that it requires a complex circuit and corresponding engineering effort, particularly in view of the cascading equalizers.
A circuit is disclosed in the IEEE publication Aerospace and Electronic Systems, Vol. 20, No. 6, November 1984, pages 821-824, entitled, “A Simple Method of Sampling In-Phase and Quadrature Components”, which mixes down the reception signal, band limits the same, samples and performs analog-to-digital conversion. The digital signal is downsampled and fed to two parallel switched all-pass filters, of which the one emits the real signals component and the other the imaginary signal component of the input signal.
This circuit is disadvantageous due to its very high sampling rate, which requires a high processing speed in the analog-to-digital converter. Furthermore, the two fifth order all-pass filters require a complex switch.
The prior art DAB systems described above fail to provide for a means for digital content providers to send targeted data to specific receivers based on information gathered at the transmission site. This function can be accomplished using a Push-Pull gateway architecture in conjunction with the present invention. The current inability of DAB content providers to target the delivery of data differs from the present invention. The present invention includes the ability to enable content providers to “Push” their digital data to specifically targeted receivers.